One type of conventional water purifier employs air to strip volatile (low boiling point) organic contaminants from water. Historically, air discharged from the purification was simply discharged to the atmosphere. Because this air is contaminated, this is no longer acceptable in many applications. The air also must be cleansed and the contaminants properly disposed of. An accepted and practical solution is to flow the contaminated stripper air through or across an adsorbent bed such as activated carbon, hereinafter referred to as carbon, that will properly capture the contaminants. However, since the air is stripping contaminants from water the air is also laden with moisture. This limits the effectiveness of the carbon because the moisture occupies many of the adsorption sites on the carbon which ideally would be occupied by the contaminant specie, e.g., trichloroethylene (TCE). In order to improve the efficiency of such systems the air may be preheated before entering the bed to reduce the relative humidity, but this requires a special heater and added energy to heat the large mass of air flowing through the bed.
After a time, the bed is loaded with the volatile organic contaminant (VOC) and moisture and becomes inefficient; the bed must then be regenerated, by heating, for example to 100.degree. C., to drive off both the volatile contaminant and the moisture. When the carbon bed is used to clean liquids contaminated with heavier, more complex organic molecules, the bed must be heated to even higher temperatures, e.g., 800.degree. C., for regeneration. At these elevated regeneration temperatures (800.degree. C.), a special furnace is required, and the regeneration is not normally done in situ: the entire bed is transported to the high temperature furnace for regeneration and then returned and reinstalled to resume operation, a costly and time consuming procedure.
The principal operating cost of the activated carbon process is the cost of regeneration. Steam is the most common means of regenerating carbon. Water is first heated to steam in a boiler and the steam is then passed over or through the carbon. As the steam heats the carbon to its regeneration temperature, the VOCs are released and flushed away by the steam. The VOCs are recovered either in a mixture of water via condensation or separately via condensation and VOC/water separation (decantation or distillation). The process of heating the carbon with steam is inefficient because the heat is transferred indirectly first to the water to make steam and then to the carbon via the heat capacity of the steam. Steam systems also require expensive distillation equipment needed to separate the VOCs from the water, which further increases the energy requirements.
Another method of regenerating the activated carbon recently has been introduced and involves the use of a dry, inert gas, such as nitrogen, for solvent recovery applications. The inert gas is first heated above the regeneration temperature of the carbon and then passed over the carbon bed. As the carbon bed heats up, the VOCs desorb from the carbon and are purged from the carbon bed in a manner similar to steam. The major advantage of this method is that because the inert gas is dry, the VOCs can be recovered directly by cooling and condensation without any water/VOC separation. The operating costs of this system can be lower for the removal and separation of water soluble solvents where other evaporation steps are required, e.g., distillation. However, because the heat capacity of the inert gas is lower than for steam, more gas is required to heat the carbon, from a point of view of both volumetric flow rate and flow time. As a result, the capital cost of the VOC recovery system for inert gas is much higher than for steam.
With respect to adsorption, although the problems have been explained in terms of water being the specie that it is desired to inhibit from occupying adsorption sites on a carbon bed so that TCE can be more effectively collected, the problem is broader than that. It can occur between any two or more species using a suitable material for the decontaminating sorbent bed.
One prior art system, disclosed in the Tigglebeck et al. patent, U.S. Pat. No. 5,187,131, teaches in situ regeneration; however, this system utilizes a technique similar to the prior art inert gas technique described above. Using an inert gas to purge the carbon bed is energy inefficient and increases operating costs.
Another prior art system, disclosed in the Rintoul European Patent Application No. 83306795.4, teaches remote as opposed to in situ regeneration of activated carbon. This system also requires the use of an inert gas or steam to purge the contaminants from the carbon.